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Nurse Mario Daponte prepares to give Melissa Cutler, 31, her H1N1 flu shot in Ottawa during the 2009 outbreak. A new study shows the vaccine is perfectly safe for developing fetuses — and may have unexpected health benefits for a woman's unborn child.

Photograph by: Wayne Cuddington
, The Ottawa Citizen

OTTAWA — Pregnancy is no time for women to refuse a flu shot since it may bring unexpected benefits to their unborn children, says a new study that analyzed vaccination rates during the 2009 H1N1 pandemic.

In addition to protecting pregnant women from the flu, the vaccine also may lower the risk of premature births, stillbirths and underweight babies, according to the study by an Ottawa research team.

Evidence is growing, based on the findings of this study and a few others, that flu vaccines are safe and effective, not only for expectant mothers, but also for their unborn infants. While these groups stand to benefit most from flu vaccines, little research has been conducted on how they respond to them.

Researchers say the latest findings could help debunk persistent myths about the flu shot and its effects on pregnant women — people who, as a group, remain reluctant to get the shot.

"The most important thing about this study was that the flu vaccine did no harm to the fetus," said Dr. Mark Walker, an obstetrician at the Ottawa Hospital and the study's co-author. "Not only was the vaccine safe, there was a potential benefit."

Walker and his collaborators studied the records of 55,570 Ontario mothers, 23,340 of whom were vaccinated during pregnancy from November 2009 through April 2010. Compared with unvaccinated mothers, women who got the shot during the H1N1 pandemic had fewer preterm births and stillbirths and fewer underweight babies, the study found.

The findings held even after researchers adjusted for factors that could skew the results, including age, education and income levels, as well as whether the mothers smoked, had hypertension or suffered from other chronic illnesses.

For every 1,000 vaccinated mothers, there were 2.6 stillbirths compared to 4.3 among unvaccinated moms — a 34 per cent decreased risk for vaccinated women.

Likewise, there were 83.3 underweight babies for every 1,000 vaccinated women, compared with 98.2 among those unvaccinated, translating to a 19 per cent decreased risk.

And there were 6.1 premature births for every 1,000 vaccinated women, compared with 8.4 among those unvaccinated, leading to a 28 per cent decreased risk.

The study, published in the June issue of the American Journal of Public Health, was a collaboration between the Ottawa Hospital Research Institute, the Children's Hospital of Eastern Ontario Research Institute and BORN Ontario, the provincial maternal-child health registry.

For Walker and his collaborators, the vaccination data from the H1N1 pandemic proved useful because, for the first time, they had information on a large enough group of pregnant women to compare the vaccine's effects on birth outcomes.

In normal years, so few expectant mothers get the seasonal flu vaccine that researchers lack a large enough sample size to conduct reliable studies.

During the pandemic, health authorities identified pregnant women as the group most threatened by the H1N1 virus and most in need of the vaccine. "Pregnant women, for whatever reason, were much more susceptible to H1N1 and when they did contract it, got much sicker than the average person," said Walker.

Even so, only 42 per cent of expectant mothers in Ontario got the vaccine.

Previously, a small-scale clinical trial showed that pregnant women who received a flu vaccine provided immunity to their newborns as well. The finding was significant because existing flu vaccines have little effect on babies in their first six months of life.

The 2008 study, published in the New England Journal of Medicine, was the first to show that babies whose mothers got the vaccine were 63 per cent less likely to catch influenza compared with the controls. Experts say such immunity typically lasts until a baby reaches six months. After that, additional flu shots are needed to once again build up an infant's immune response.

Nurse Mario Daponte prepares to give Melissa Cutler, 31, her H1N1 flu shot in Ottawa during the 2009 outbreak. A new study shows the vaccine is perfectly safe for developing fetuses — and may have unexpected health benefits for a woman's unborn child.

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